1. Field of the Invention
The present invention relates to a signal processing circuitry for a distance measuring system.
2. Description of the Prior Art
Various kinds of devices for measuring a distance to the object have been proposed.
Such distance measuring devices can be roughly classified into two types, so called passive system and active system. The former system measures the distance by receiving the light from an object without projecting any illumination beam and converting the light into an electrical signal so as to detect the maximum value which corresponds to the distance to the object, while the latter projects a light beam to an object and detects the maximum value of reflected light from the object through an electrical means.
In the latter active system, the incident light amount upon the distance measuring device reflected on the object has very wide range when supposed that the objects to be measured lie at 0.8 to 8 meters with the reflection coefficient of 10 to 100 (%). Namely, the incident light amount is proportional to the reflection coefficient and the inverse-square of the distance so that it varies by 1000 times [=(8/0.8).sup.2 .times.100/10], which means the maximum signal reaches as high as 10 (v) supposed that the minimum is 10 (mv) in the active system if the processing circuitry has unity gain.
Even in case of the above mentioned passive system, the variation range of the ambient brightness of the object is, as is well known, more or less equal to that of the case of the active system, so that the input signal reaches as high as 10 (v), in case the minimum signal is 10 (mv).
Consequently, the conventional active and passive distance measuring system should include the circuitry with a very wide dynamic range to process the light-information so that the circuitry requires a high voltage source, which is large and inconvenient for a small optical instrument.
Although a method of compressing input signal has been proposed in order to narrow the dynamic range, whereby if this method is adopted in a distance measuring device so as to compress the input signal uniformly, the input signal corresponding to the light reflected from an object at a long distance is compressed in the same way as that from an object at a short distance so that especially the detecting accuracy, in other words, the distance measuring accuracy of the object at a long distance is remarkably lowered, which is also very inconvenient.
Consequently, these methods cannot be applied to the distance measuring device and, therefore, it is much required that a method to narrow the dynamic range suited for the distance measuring device should appear.